1. Field of the Invention
The invention relates to a digital radio communication system, comprising at least one primary station and at least one secondary station, which stations exchange digital data in at least one time slot of a time multiplex frame via at least one frequency channel, the primary station and/or the secondary station comprising control means which are capable of replacing a first time-slot/frequency-channel combination allocated to data exchange by a second time-slot/frequency-channel combination.
The invention also relates to a primary and a secondary station for use in such a system.
2. Description of the Related Art
A digital radio communication system of this kind is known from an article in Philips Telecommunication Review "DECT, a universal cordless access system", Vol. 49, No. 3, September 1991, pp. 68-73. In the known system digital data is exchanged, for example in the form of digital coded speech or data, between a primary DECT station (DECT, Digital European Cordless Telecommunication standard), being a so-called base station, and a secondary station, being a so-called handset. In DECT, being a cordless telephony system based on a cellular concept, there are 10 frequency channels via which radio communication takes place, in principle in the duplex mode in time slot pairs, between the base station and the handset. In DECT, per DECT-TDMA frame of 10 ms there are defined 24 time slots, i.e. 12 for radio traffic from the base station to the handset and 12 for radio traffic from the handset to the base station, said time slots forming 12 time slot pairs for each frequency channel. Consequently, there are 120 duplex time-slots/frequency-channel combinations or 120 channels in DECT. In a system such as DECT, in which the physical network layer implements an FDMA/TDMA/TDD radio transmission, the channel allocation is dynamic, i.e. if channels are required for radio communication, they are dynamically allocated from a set of available channels and, after termination of the communication, they are returned to the set of available channels. Furthermore, because of the frequently changing receiving conditions, a comparatively large number of so-called "handovers" occurs, i.e. the switching over of a radio communication in progress via a given channel to another channel. In the case of interference, such a handover may be intra-cellular or also, when the handset leaves the range of the base station, inter-cellular. The handover, taking place, for example in the event of a poor transmission quality, is initiated by the handset or the portable set in DECT. The DECT standard calls for such a handover to be seamless, i.e. so that a user of DECT does not notice the transition from a first time-slot/frequency-channel combination to a second time-slot/frequency-channel combination since radio communication is allowed via two different time slot pairs for a number of frames, but the cited standard does not make an implementation proposal.